So-called container "panelling" is a term of art meaning that the normally straight or flat sidewalls of a plastic fluid container are distorted inwardly, presenting an undesired appearance to a consumer and giving rise to consumer uncertainty as to content quality.
By way of an understanding of a cause of panelling, consider that a liquid container has part product content, e.g., a liquid, and part air content, i.e., volume of the container not occupied by the liquid product, or head air space. If, after sealed closing, the original air volume at container closure contracts in volume, it accordingly increases in vacuum pressure. Such increased vacuum pressure in the container can collapse sidewalls, i.e., the excessive pressure is not realizable without container volume decrease. Thus, there occurs panelling, container volume decrease and accommodation of pressure change.
Panelling is not a matter of concern where the container sidewalls are, by nature, tolerant of original air content volume contraction and vacuum pressure attending same. Thus, glass and metal containers generally are not susceptible to panelling and do not present the aforementioned consumer acceptance problems associated therewith. However, plastic containers in widespread use are susceptible to panelling and frequently evidence same. By way of example, assume automotive motor oil at an elevated temperature, substantially above ambient/environmental temperature, to be containerized at such elevated temperature. With a proper closure seal presumed, upon drop in temperature of the motor oil to ambient temperature, panelling occurs due to lessening of head space air volume and increased vacuum pressure attendant thereto. Accordingly, measures, costly and time-consuming, need be taken to avert such panelling. Typically, there is believed to be a forced cooling of the motor oil prior to containerization. Such anti-panelling measure is disadvantageous, both in terms of the cost of the practice, the inconvenience of container sealing at a time delayed from filling and attendant shipment delays. Panelling also limits the stack load possible due to unbalance bottle supportability caused by panelling.